This invention relates to a knock detector for detecting the occurrence of knock in a spark-ignited internal combustion engine.
The phenomenon of engine knock has long been known to those familiar with internal combustion engines. Engine knock is outwardly manifested by the occurrence of an audible pinging sound. The severity of the knock is reflected by the intensity and/or duration of the pinging sound. Automotive internal combustion engines are designed to operate on fuel of a specified minimum fuel octane rating, and in general an occasional light transitory knock is not objectionable in an internal combustion engine in an automobile. Indeed, compression ratios are selected and spark advance schedules are advanced for automotive internal combustion engines for the purposes of maximizing fuel economy to an extent where under transient operating conditions an occasional light knock may occur. However, more intense and/or more prolonged occurrences of engine knock are definitely undesirable as they have the potential for greatly increasing wear and tear on the engine and even causing major damage to engine-related components. Thus, while an engine may be designed on paper and constructed in the laboratory for knock-free operation, the variables which come into play in actual mass production of a spark-ignited internal combustion engine and in actual use of the engine in an automotive vehicle can give rise to objectionable levels of engine knock. Non-uniformity of gasoline octane, changing ambient conditions, and variations in manufacturing tolerances are among the factors which individually and collectively can adversely affect the engine operation and create knock.
If knock does occur, a proven technique for its alleviation involves an appropriate retarding of the spark timing. Unfortunately, this technique is inconsistent with the objective of increasing fuel economy.
The advent of electronic engine controls on production automotive vehicles offers the possibility for selectively retarding spark timing in response to the detection of incipient, or borderline, engine knock. For example, current electronic spark advance systems electronically adjust the timing of spark ignition as a function of conventional variables such as engine speed and engine load. By adding a suitable knock detector to such a system knock can be detected at borderline levels and spark timing can be automatically retarded to alleviate the knock and thereafter advanced as otherwise scheduled. This means that the fuel economy and emissions benefits can be obtained while knock and its related problems can be avoided.
The present invention in one respect is directed to the provision of such a suitable knock detector.
A study of a number of previous patents reveals that in a general way, knock detectors may be divided into two categories. The first category involves those knock detectors which are utilized to grade and/or blend gasoline. The second category involves those which are employed in actual usage in an automotive engine for over-the-road operation. Examples of patents in the first category are U.S. Pat. Nos. 3,469,954; 3,575,039; and 3,822,538. An example of a patent of the second category is U.S. Pat. No. 4,002,155. Knock detectors in the first category are designed for what basically amounts to a controlled laboratory situation where engine speed may be accurately controlled, where accelerations and decelerations may be slowly, uniformly made, and where fuel octane may be accurately monitored. Such conditions are in marked contrast to the highly dynamic operation of an engine in an automobile where the engine is subject to sudden accelerations and decelerations, a wide range of operational speeds, and where shift points of the transmission coupled to the engine impose sudden extreme transients on the engine, not to mention of course the nonuniformity in octane ratings of gasolines vended at the pump. It is a far easier task to detect engine knock under controlled laboratory conditions than it is in an automobile operating on the road. One problem with previous knock detectors of this first category is that they would appear generally unable to consistently and accurately detect knock if the engine were subjected to the more severe operating conditions to which it would be subjected if in an automobile.
The knock detectors falling into the second category operate to retard spark in response to incipient knock, but in general they seem to contain large amounts of electronic circuitry, and for many of these knock detectors, it is questionable whether they can consistently and accurately detect knock under all transient conditions which may be encountered although in general they represent more resourceful approaches than those of the idyllic first category. Furthermore, mass production requirements dictate that such systems be cost effective, reliable, and adaptive.
The present invention is directed in a further respect to fulfilling a need for a knock detector suitable for mass production which will reliably detect knock in its incipiency under the myriad of different operating conditions which may be encountered in an automobile engine in over-the-road use and which in conjunction with an electronic spark advance system will operate to retard spark to alleviate the knocking tendency and therafter allow normal spark advance schedules to be resumed for fuel economy and emissions purposes once the knocking as ceased.
Investigations of the engine knock phenomenon have demonstrated that for a given engine model knock-induced vibrations will occur at reasonably well-defined frequencies. For example, in a four-stroke reciprocating piston engine the frequencies at which knock occur appear to be functions of the diameters of the engine cylinders to a first order and the displacement to a second order, and it does not appear that they are functions of knock intensity. Thus, sensors for detecting knock may be mounted on the engine and tuned to monitor the frequency bands in which knock vibrations may be expected. The problem, of course, in detecting knock-induced vibrations at these frequencies is to distinguish them from other engine vibrations also lying within these bands, these latter vibrations constituting noise. Prior patents disclose various approaches to the problem of distinguishing knock signals from background noise in a variety of ways.
At constant or slowly changing engine speeds knock can be detected without sophisticated techniques because it is not as difficult to monitor and compensate for the background noise. For example, it is possible to use an integrator to average background noise and to monitor vibrations against this background noise level. Examples of such detectors would be those first three patents enumerated above. Under dynamic operation, however, more sophisticated detectors are called for because the range of background noise begins to increase and overlap the range of knock signals thereby tending to increasingly mask any engine knock which may occur. Patents of the second category referred to above contain these more sophisticated approaches. While offering various degrees of improvement, many still suffer from the disadvantages described above such as not being able to detect knock under extreme transient conditions or being too expensive, etc. On the other side of the coin, care must be taken in the design of a knock detector to avoid false retarding of spark timing when knock is not in fact present. False retard could be a potential problem with certain types of systems. Thus, it must be appreciated that attainment of a perfect or near-perfect knock detector is not an easy task.
Earlier patents disclose a variety of possible approaches to compensating for background noise. One approach disclosed in U.S. Pat. No. 3,393,557 involves the provision of a knock band channel and an engine noise band channel each of which contains its own band pass amplifier and peak reading circuit. The outputs from the two peak reading circuits are monitored by a differential amplifier to provide an indication of knock. A possible objection to this approach is that it presumes that noise in the passband of the noise channel is the same as noise in the passband of the knock channel and such may not be, and indeed probably is not, the case.
U.S. Pat. No. 2,958,317 incorporates a threshold circuit which is set higher than the valve noise level. Only signals exceeding the threshold are monitored. A problem with this approach is that it is obviously not suited to accommodate the variety of range of conditions which would be encountered in an automobile engine in actual use in an vehicle.
Other patents set a threshold as a function of engine speed; for example, see U.S. Pat. Nos. 4,163,385 and 4,061,116. In U.S. Pat. No. 4,002,155 a background noise level reference is used to establish a threshold and the number of ringings occurring in the vibration signal which exceed the threshold are used to determine whether knock is occurring. This latter patent is also an example of a gated reference where the vibration signal is gated during a portion of the operating cycle when knock vibrations are not expected to occur to thus establish a reference. Other approaches may employ a gated knock where the vibration signal is gated during selected portions of the cycle when knock is expected to occur; for example, see U.S. Pat. No. 2,879,665.
One patent which seems to offer a possibly more effective approach than some of the other patents is U.S. Pat. No. 4,153,020 where the gain of an amplifier receiving the vibration signal is controlled as a function of background engine noise. However, a possible problem is that it may become less effective as the engine becomes worn.
Other approaches are shown in U.S. Pat. Nos. 3,678,732; 3,950,981; 3,822,583; 4,111,035; 2,450,882, and 3,942,359.
The present invention is addressed to a new and improved knock detector which compensates more effectively to unmask engine knock in the presence of engine noise over the dynamic operating ranges encountered in an automobile engine, yet which is also highly immune to false retard of spark timing. While some of the individual techniques embodied in the practice of the invention are per se known in the art, the invention does comprise a novel means which compensates for the adverse influence of background noise on knock detection and which is well suited for an automobile engine on a mass production basis although obviously it is also useful for any type of engine knock detector. The invention is cost effective, is less complicated in circuitry, and very importantly, it achieves a high degree of consistency in accurately detecting incipient knock. Principles of the invention can be applied to various engine models with any required differences in scaling being made by selection of circuit component values. Although a preferred embodiment of the invention is herein disclosed, its principles may obviously be implemented in other embodiments.
In an automobile a constraint on electrical and electronic systems is the available voltage supply. Typically, in today's automobiles, the voltage supply is a 12 volt DC battery. Thus, electronic systems in the vehicle must operate on a supply voltage of somewhat less than 12 volts unless other techniques, such as voltage doubling, are employed. The obvious disadvantage of such other techniques is that they add to the cost and complexity of the system. A typical supply voltage for state of the art electronic circuitry is five volts. This means that in an automobile engine knock detector using such a supply voltage, knock and noise signals being processed in the knock detector must be confined to this limited range over the engine vibrating range if useful information is to be obtained; i.e. saturation must be avoided. Prior systems have utilized integrators, averagers, and/or low pass filters. Some have used variable attenuators. Yet, these approaches are not seen to be capable of providing reliable, accurate knock detection in automotive usage. They may either miss knock or they may falsely detect it.
A very desirable way to detect knock, disregarding background noise for the moment, is by utilizing peak detection. Peak detection allows even a single individual knock vibration to be detected thereby offering the potential for knock to be suppressed in its very incipiency. Peak detection, per se, in the context of an engine knock detector is well known as evidenced by the aforementioned U.S. Pat. No. 3,393,557. Now peak detection by its very nature involves detecting peak voltage. In the laboratory large voltage supplies are readily available and use of peak detection is not foreclosed by voltage supply constraints. Unfortunately, with the overlap in noise and knock signals ranges in an automobile, the use of peak detection appears unattractive in view of the low supply voltage.
The present invention, in a still further respect, makes feasible usage of the peak detection technique in a low voltage system in an automobile, encompassing the range of operational speeds and transients which may be encountered, reliably and accurately detecting knock in its incipiency to retard spark timing and yet allowing spark to be otherwise advanced in timing in accordance with desired schedules at such times as when knock is absent.
While the claims at the conclusion of the ensuing specification define the patented invention, the improvement which the invention provides may be briefly summarized as comprising in a knock detector: a sensor mounted on the engine, tuned to sense mechanical engine vibrations within a frequency band which will contain knock vibrations if knock occurs, and providing a corresponding electrical signal; an amplifier for amplifying the sensor electrical signal; a gated reference channel which receives the amplified signal and gates it to a detection circuit during intervals coincident with the absence of any appreciable engine knock with the detection circuit detecting a selected characteristic of the gated signal; a knock channel which couples the amplified signal to another detection circuit which detects a selected characteristic of the signal; a comparator which compares the detected signals detected by the two detection circuits to provide a knock detection signal in accordance with a predetermined relationship between the two indicative of engine knock; and means for sensing engine speed and attenuating, in accordance with engine speed, the gain of the amplifier such that the gain is in general made an inverse function of engine speed. In the disclosed preferred embodiment the improved knock detector of the invention comprises: a resonantly tuned sensor mounted on the engine and tuned to a frequency band in the vicinity of the six kilohertz range; an amplifier comprising a variable gain operational transconductance amplifier stage for amplifying the sensor signal; a sensor which senses engine speed and a frequency-to-current conversion circuit which converts the sensed frequency representing engine speed into a control current for the operational transconductance amplifier stage which in general increasingly attenuates the gain of the stage with increasing engine speed; a reference channel circuit including a gate which operatively couples the output of the amplifier to a peak detector circuit for peak detecting the amplified sensor signal during gating intervals which are coincident with the absence of any appreciable engine knock; a knock channel circuit which operatively couples the output of the amplifier to another peak detector circuit for peak detecting the amplified sensor signal; and a comparator circuit for comparing the outputs of the two peak detector circuits to provide a knock detection signal in accordance with a predetermined relationship between the two indicative of engine knock. The characteristics of the operational transconductance amplifier and frequency-to-current conversion circuit are selected such that the knock sensor signal is amplified by a factor of eight for engine speeds up to 1000 rpm and between 1000 rpm and 4000 rpm, the gain of the amplifier varies linearly inversely with increasing engine speed to arrive at an ultimate amplification factor of unity at 4000 rpm and remains at unity beyond 4000 rpm. The knock detector may be employed in conjunction with either an analog spark advance system or a digital spark advance system whereby spark timing is retarded in response to detection of incipient knock and thereafter normal spark advance schedules are allowed to resume once the knock has dissipated.
The foregoing features, advantages, and benefits of the present invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings disclosing a preferred embodiment of the present invention according to the best mode presently contemplated in carrying out the invention.